The present invention relates to a power semiconductor device such as a diode and thyristor.
In such a system which handles a large power as a power converter or inverter, a high breakdown voltage, large current capacity semiconductor devices are used. As a high breakdown voltage semiconductor device, there are such devices as a diode, thyristor, GTO thyristor, reverse conducting thyristor and the like which has a laminated structure of a plurality of p-type and n-type semiconductors.
In these semiconductor devices, it is desirable for both characteristics of ON-state voltage V.sub.T applied across both sides of a semiconductor device in conduction state, and of reverse recovery charge Q.sub.r which is a total charge which flows in the reverse direction at the time of switching from the conduction state to the blocking state to be substantially small. Lifetime of carriers within the device is one of the major factors that determine these two characteristics described above. The longer the lifetime the lower the ON-state voltage V.sub.T becomes while the shorter the lifetime the smaller the reverse recovery charge Q.sub.r becomes, thereby, these two characteristics are in a trade-off relationship from each other.
As means for controlling the lifetime, there are known methods such as diffusion of heavy metals such as gold, platinum or the like, and irradiation of radiation rays. In the case where lifetime in a device is shortened by irradiation of .gamma. rays or electron beam, its lifetime in the depth direction of the device is uniformly shortened. Thereby, although its reverse recovery charge can be reduced, an increase in its ON-state voltage cannot be avoided. As means for improving these characteristics in semiconductor devices by locally controlling the lifetime, there are disclosed prior art in JP-A-57-39577 and JP-A-60-198778.
The prior art of JP-A-57-39577 discloses a method of irradiating protons from the main surface of a thyristor to form lattice defect in a region in which a depletion layer expands in the vicinity of pn junction on the anode side to which a reverse voltage is applied at the time of turn-off operation such that lifetime in the device is locally shortened. Further, the prior art of JP-A-60-198778 discloses a method of locally shortening lifetime in one of two regions which form pn junction to which a reverse voltage is applied at the time of turn off switching, and in particular, in an area of the one of two regions having a smaller impurity concentration in the vicinity of the junction where a depletion layer expands.
Both of these prior arts are directed to reducing reverse recovery charge Q.sub.r while suppressing an increase of ON-state voltage V.sub.T to a small value by rapidly reducing carriers in the region where a depletion layer will expand at the turn off switching operation so as to be able to form the depletion layer rapidly.
FIG. 5 shows a result of calculation of a distribution of carrier density in conduction state made by the present authors in a case where lifetime in the vicinity of pn junction on the anode side is locally shortened according to the prior art of JP-A-57-39577. By way of example, in the drawing of FIG. 5, a position at 0 .mu.m is a surface of n-emitter layer on the cathode side, and a position at 1520 .mu.m is a surface of p-emitter layer on the anode side (the same applies to FIG. 6). Further, a distribution of impurity concentration used in the calculation is shown in FIG. 2, which will be explained later (the same applies to FIG. 6). In this case, since a carrier density in a region in the vicinity of pn junction on the anode side where its lifetime is locally shortened is lowered, thereby, since a resistance component in inverse proportion to the carrier density increases, ON-state voltage V.sub.T will increase consequently.
Further, FIG. 6 shows a result of calculation of a distribution of carrier density in conduction state made by the present authors in a case where lifetime in an n-base region is shortened according to JP-A-60-198778. In this case, since a carrier density in conduction state in the p-emitter layer is high, reverse recovery charge Q.sub.r is not reduced effectively.